Ionic compounds are typically crystalline solids that have a high melting point. These properties of ionic compounds derive from strong electrostatic interactions between ions of opposite charges, which provides a large enthalpic driving force for the formation of ordered lattices (in which lattices the ionic interactions can be maximised).
However, there are certain ion-based systems that remain in the molten state at relatively low temperatures (e.g. about ambient temperature), and are therefore termed “ionic liquids”. The fluid state in these systems is typically maintained by the use of cations and/or anions that:
(a) are based upon, or contain, an organic molecule; and
(b) have a large radius.
The large radius of the cation and/or anion decreases the magnitude of the charge-charge interaction between the ions, thereby reducing the driving force for the system to convert to an ordered lattice. Also, in these systems, the cations and anions are almost always mono-charged, so as to further minimise the ionic interactions.
Ionic liquids have a number of useful and interesting properties. For example, due to their characteristically very low vapour pressure, they represent an attractive alternative to conventional (uncharged) solvent systems, which can produce hazardous vapours. Further, they have particular application as electrolytes or as solvents for ionic compounds (such as metal salts) and thus have utility, for example in electrochemical systems (such as fuel cells, electrochromic devices and photovoltaic devices) and electrochemical processes (such as electrodeposition and electropolishing).
In the known ionic liquids, various approaches are utilised to form the requisite large cations or anions.
For example, a metal salt that contains a Lewis acidic metal ion can be reacted with an ionic compound that contains a large, organic cation and an anion that can coordinate to the metal ion. In this way, a fluid system is formed that contains a metal-based anion and the organic cation. Examples of such systems include:    (a) trimethylphenylammonium chloride or 1-ethyl-3-methylimidazolium chloride/AlCl3 (as described, for example, in U.S. Pat. No. 4,764,440, U.S. Pat. No. 5,525,567, FR 2 611 700, FR 2 626 572, EP 0 838 447 and WO 95/21872), which provide liquids containing AlCl4− anions and either imidazolium or trimethylphenylammonium cations;    (b) tertiary ammonium halide or quaternary ammonium or phosphonium salt/metal halide, where the metal is, for example, zinc or aluminium (as described in U.S. Pat. No. 5,731,101 and U.S. Pat. No. 5,892,124); and    (c) quaternary ammonium halide/optionally hydrated metal salt, where the metal is, for example, iron, zinc or tin (as descried in WO 00/56700 and WO 02/26381) and where the resulting ionic liquid is relatively water-insensitive.
Another approach is to generate a large anion by utilising a molecule that “solvates” (i.e. hydrogen-bonds to) the anion of a crystalline ionic compound. Thus, for example, it is know that urea, which has hydrogen bond donor properties, can be mixed with highly ionic halide or nitrate salts of alkali metals to provide an ionic liquid (as descried, for example, in Thermochim Acta 111, 37-47 (1987) and ibid. 127, 223-236 (1988)). Such ionic liquids have been utilised in electrochemical processes (e.g. the electroreduction of Co(II) or Ni(II), as described in Rare Metals 19(3), 237-241 (2000)).
Other known ionic liquids containing hydrogen bond donors (such as urea) include those described in WO 02/26710, which are formed by reaction between a quaternary ammonium salt and a hydrogen bond donor. The use of such ionic liquids for the dissolution of metal salts and oxides is also described.
Although urea and thiourea are known to interact with various metals and anions in solution (see, for example, Science 123, 897 (1956) and J. Am. Client. Soc. 79, 4296-4297 (1957)), there has, to the knowledge of the applicant, never been a suggestion that it would be possible to prepare an ionic liquids by reacting urea with a salt of a multiply charged metal ion.
Indeed, it would be counter-intuitive to attempt to prepare such an ionic liquid, as the metal ion either:    (a) in highly ionic salts (e.g. those of alkaline earth metals) would be likely to dissociate from the anions, thereby forming a multiply-charged cation (which would have increased charge-charge interactions with the surrounding anions, thus providing a greater driving force for the formation of an ordered lattice, i.e. crystalline material); or    (b) in less ionic salts (e.g. those of the transition metals) would be unlikely to dissociate from the anions, thus not providing the separation of cations and anions required for the formation of an ionic liquid.
The applicant has now surprisingly discovered that ionic liquids may be formed by mixing a neutral organic molecule such as urea with a metal salt that is weakly ionic and/or that contains a multiply-charged metal ion.